As is apparent from the aforementioned copending application, stacked plate friction devices generally comprise a shaft rotatable in a housing, each being coupled to one or more plates of a stack of interleaved axially shiftable plates which can be pressed by a pressure plate in the housing against a counter surface of the latter when actuation of the brake or the clutch constituting the friction device is desired.
German Patent document DE-PS No. 2802 676 describes a forced cooling friction device of this type which is provided with means for controlling the coolant flow to a cavity of a plate carrier on the shaft from which the coolant flows radially outwardly into the stack, a control means being provided to regulate this flow so that in the inoperative position of the pressure plate the flow is blocked, but with displacement of the pressure plate to compress the stack, the flow is generated to follow the loading of the friction device.
In this system, however, the flow of the coolant is effected through the shaft which requires control of the outflow in the region of the shaft surrounded by the stack and the carrier. It has been found that this construction is impractical because sealing difficulties arise from the tolerance problems which result from having the coolant oil passages in the shaft, i.e. a rotary element.
Furthermore, because of the fact that the passages are provided in the rotary shaft, the supply of the coolant fluctuates in accordance with the speed of the device which is often a handicap.
Finally, it has been found, with such systems, that the supply of the coolant oil differs from plate to plate along the stack because of the use of an obstructive formation to control the flow and this is, of course, undesirable. Indeed this arrangement has other problems which make it unreliable including interference uniform to outflow of the cooling oil.
These problems are not eliminated in other cooling systems for friction devices, for example, in EPS No. 143898, the use of large brake disks with a number of large passages between the friction lining and the inner plate carrier is proposed, but this not only inordinately increases the cost for the plates, but also gives rise to increased oil spraying and uncontrolled cooling oil flow so that the lining wear is nonuniform.
Coaxial cooling oil passages are eliminated in the plates themselves in the system of U.S. Pat. No. 4,529,073 in which a cavity or chamber or compartment is provided as in the German Patent document No. 2802676 as a coolant oil collector before the coolant oil is permitted to flow into the plate stack. However, in this arrangement, the control valve is also in a rotating member, i.e. a rotating drum, and this operates with an undesired speed dependency so that its operation is not coordinated with the small movements of the brake pressure plate.